The present invention relates to sensor arrays for sensing electromagnetic radiation, and in particular, to active pixel sensor arrays.
Image sensors and other light sensitive sensors may be fabricated to detect the intensity of light received by the sensor. These sensors typically generate electronic signals that have amplitudes that are proportional to the intensity of the light received by the sensor. The sensors can convert an optical image into a set of electronic signals. The electronic signals may represent, for example, intensities of light received by the sensor. The electronic signals can also be conditioned and sampled to allow image processing.
One of the currently available types of image sensors is commonly referred to as an active pixel sensor. Active pixel sensors are typically fabricated using standard (CMOS) processes enabling these sensors to be integrated with digital and analog signal processing circuitry.
In conventional active pixel sensors, each pixel cell typically comprises photosensitive and non-photosensitive devices. The types of photosensitive devices include photodiodes, photoconductors, photogate MOS capacitors, and other similar devices. Non-photosensitive devices that may be found in many active pixel sensors include one or more transistors.
In many active pixel sensors, the photosensitive devices compete with non-photosensitive devices for available space on the sensor. Advances in CMOS processes permit the fabrication of sensors having pixel cells with increasing smaller geometries. As a result, the junction depth of the PN junctions and the depletion width of the MOS capacitors shrink proportionally. However, many CMOS fabricated sensors contain junction depths that are so shallow that they become much smaller than the absorption length of visible light in silicon substrate. As such, conventional active pixel sensors may suffer from deteriorating photosensitivity that may be proportional to the shrinking of the baseline CMOS process.
A variety of existing active pixel sensors are fabricated by layering a translucent conductive layer over a PIN or NIP photodiode, which is formed over a substrate. Typically, the bottom layer of the PIN or NIP photodiodes is connected to a pixel electrode that is associated with an individual pixel in the pixel cell array.
In some sensors, a voltage is applied to the top transparent conductive layer to reverse-bias the PIN (or NIP) photodiode. In conventional three-transistor pixel cells, for example, the pixel electrode is electrically shorted with the charge-collecting node of the pixel cell. Thus, during the charge integration process, the electrical potential of the pixel electrode may vary from pixel to pixel, depending on the amount of charge collected at each pixel site. In many sensors, each of an array pixel cells may have pixel electrodes that are electrically connected because they all share a common bottom layer of the PIN or NIP diode.
A problem that typically occurs when neighboring pixel electrodes are not electrically isolated from one another is commonly referred to as pixel crosstalk. Pixel crosstalk may occur in conventional active pixel sensors when current flows from higher-potential electrodes to neighboring, lower-potential, electrodes. The presence of pixel crosstalk is often undesirable because it may result in the capturing of a blurred image.
While there have been some attempts to design sensors that alleviate or minimize undesirable affects, such as pixel crosstalk, these attempts have not been entirely successful. Accordingly, a present need exists for an active pixel sensor that can provide, for example, increasingly sharper images by minimizing undesirable affects such as pixel crosstalk.
The active pixel sensor of the invention includes, in one embodiment, a solid state radiation detection unit comprising a crystalline semiconductor substrate, a plurality of complementary metal oxide semiconductor (CMOS) pixel circuits incorporated into the substrate to form an array of pixel circuits. Typically, each of the array of pixel circuits include a charge collecting pixel electrode, a charge sensing node, and a gate bias transistor separating the charge collecting pixel electrode and the charge sensing node. Each pixel circuit may further include a pixel capacitor to store charges collected by the charge collecting pixel electrode. A charge measuring circuit comprising at least one transistor may also be configured with each pixel circuitry. Typically, a gate of this transistor is electrically connected to the charge sensing node. A radiation absorbing layer comprised of photoconductive material typically covers at least a portion of the array of pixel circuits, while a surface electrode layer comprised of electrically conducting material may be formed on the radiation absorbing layer. Typically, the surface electrode layer is at least partially transparent to the electron-hole producing radiation and may be connected to a voltage source for establishing an electrical field across the radiation absorbing layer and between the surface electrode layer and each of the array of charge collecting pixel electrodes. The sensor may also be configured with an array measurement circuit for measuring charges collected by each of the array of charge collecting pixel electrodes, and if desired, for outputting pixel data indicative of the collected charges.
In accordance with one aspect of the present invention, each of the array of pixel electrodes may be maintained at substantially equal potential by the gate bias transistor.
In accordance with another aspect of the present invention, a gate of the gate bias transistor may be biased by constant voltage to minimize pixel crosstalk among adjacent pixel electrodes within the array of pixel electrodes.
In another aspect of the present invention, the charge sensing node comprises metal and provides an electrical connection to the gate of the at least one transistor in the charge measuring circuit.
In still yet another aspect of the present invention, the charge sensing node comprises polycrystalline semiconductor material and provides an electrical connection to the gate of the at least one transistor in the charge measuring circuit.
In another aspect of the present invention, the charge sensing node comprises a p-type doped region in the substrate and provides an electrical connection to the gate of the at least one transistor in the charge measuring circuits. Alternatively, the charge sensing node may comprise an n-type doped region in said substrate.
In still yet another aspect of the present invention, each of the array of pixel circuits may include, for example, either two, four, six, or more transistors.
In yet another aspect of the present invention, the pixel capacitor may be defined by the structure between the charge sensing node and the crystalline semiconductor substrate.
In accordance with another aspect of the present invention, the radiation absorbing layer comprises hydrogenated amorphous silicon.
In accordance with another aspect of the present invention, the radiation absorbing layer may be continuous layer, a discontinuous layer, a patterned layer, or some combination thereof.
In another aspect of the present invention, the radiation absorbing layer comprises trenches.
In still yet another aspect of the present invention, the radiation absorbing layer is substantially planar.
In another aspect of the present invention, the radiation absorbing layer is a continuous layer that is fabricated during a continuous deposition process.
In yet another aspect of the present invention, the radiation absorbing layer is a p-n photodiode layered structure, such that the p-layer is electrically connected to the charge collecting pixel electrode, while the n-layer is electrically connected to the surface electrode layer.
In still yet another aspect of the present invention, the radiation absorbing layer is p-I-n photodiode layered structure, such that the n-layer is electrically connected to the charge collecting pixel electrode, while the p-layer is electrically connected to the surface electrode layer.
In another aspect of the present invention, the radiation absorbing layer may be a n-I-p photodiode layered structure, such that the p-layer is electrically connected to the charge collecting pixel electrode, while the n-layer is electrically connected to the surface electrode layer.
In yet another aspect of the present invention, the p-layer comprises p-type doped hydrogenated amorphous silicon.
In accordance wit h another aspect of the present invention, the n-layer comprises n-type doped hydrogenated amorphous silicon.
In accordance with another aspect of the present invention, the I-layer comprises un-intentionally doped hydrogenated amorphous silicon.
In another aspect of the present invention, the radiation absorbing layer comprises a photoconductive un-intentionally doped layer.
In still yet another aspect of the present invention, the photoconductive un-intentionally doped layer comprises hydrogenated amorphous silicon.
In another aspect of the present invention, the charge collecting pixel electrode comprises a patterned metal plate.
In yet another aspect of the present invention, the charge collecting pixel electrode may be formed by a surface of at least one via used for interlayer connection by a semiconductor fabrication process.
In still yet another aspect of the present invention, the charge collecting pixel electrode may be formed by a surface of a single via.
In yet another aspect of the present invention, the surface electrode layer may comprise indium tin oxide, tin oxide, titanium nitride, or other similar materials.
In accordance with another aspect of the present invention, a potential difference between adjacent pixel electrodes may be in a range of about 1 to about 50 millivolts.
In still yet another aspect of the present invention, the sensor comprises a fill factor anywhere from about 40 to 80 percent, or even more.